We seek funding to continue our studies of the structural chemical basis of the "high energy phosphate bond." All kinds of metabolic energy exchanges are based on the ATP-ADP bioenergetics cycle in which free energy is stored by condensation of ADP and Pi and then released as needed by hydrolysis of the terminal phosphate of ATP. We are studying a series of small, model phosphates that span the biological range of free energy of hydrolysis of "phosphate group transfer potential." The series ranges from the high-energy phosphoenolpyruvate to the low-energy 3-phosphoglycerate, and includes inorganic orthophosphate and pyrophosphate as well as acyl-, guanidyl-, and sugar-phosphate species. Our principal studies are highly accurate single-crystal X-ray and neutron diffraction analyses that extend to detailed analyses of the terminal vibrations and electron density distributions in the crystals. These studies provide accurate molecular geometries, corrected for thermal vibration effects, and detailed experimental maps of the valence electron density redistributions due to chemical bonding and of the molecular electrostatic potentials. For microcrystalline compounds of interest that do not form suitable single crystal for these X-ray and neutron analyses, we propose new approaches based on a powerful combination of alternative crystallographic diffraction techniques: electron crystallography for unit cell and space group determination, state- of-the-art methods of X-ray powder diffraction for structure determination and refinement, time-of-flight neutron diffraction with powder samples and a pulsed neutron spallation source for hydrogen structure refinement, and synchrotron X-ray diffraction for very small single crystals and powder samples. Our studies will provide accurate experimental data on bond lengths, net atomic charges, and P Pi(o) to d Pi(P) partial double bond orders. Comparisons of these results for high-energy vs. low- energy and unhydrolyzed vs. hydrolyzed phosphates will clarify the roles of ionic charge delocalization and electronic resonance as determinants of the free energy changes accompanying biological phosphate hydrolyses. The very important role od hydration of the hydrolysis reactants and products will be explored by using our charge density distributions and electrostatic potentials to model structures and energetics of phosphate ion-water molecule association interactions.